White smell: the olfactory equivalent of white noise

Chanel No. 5. J’adore by Dior. Eau de White? It might not be on your wish list but it is now possible to bottle a whiff of white.

Noam Sobel and colleagues from the Weizmann Institute of Science in Rehovot, Israel, wanted to make the olfactory equivalent of white light and white noise. Because white light is made up of all the different wavelengths of visible light at roughly equal intensity, and white noise is a spread of different sound frequencies of equal power, they reasoned that white smell could be created using a mixture of different odours of the same strength.

They took 86 odours made up of a different single molecule, commonly used in the perfume industry, and diluted them so they smelled equally strong. They then created mixtures that included between one and 43 odours, that spanned a range of molecular structures and smells.

The researchers asked participants to rate how similar in smell pairs of mixtures were. The more odours two mixtures contained, the more likely the participants were to rate them as similar, even though they had no individual components in common. “This suggests that if more non-overlapping odours are added to the two mixtures, they should eventually smell the same,” says Sobel. “This would be olfactory white.”

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Mixtures with more than 30 components were perceived to be similar but the participants could still distinguish between them. “If you look at a white wall and hold up a piece of white paper, you’ll be able to tell the difference,” Sobel says, “but you’d still call both white.”

The team then created four different white mixtures of 40 components. They named them all “laurax” and assigned each to participants to smell. After four days, the people were asked to identify their laurax from 23 mixtures of different numbers of odours. The participants correctly identified their laurax every time, but half of the time they also thought that any mixture that contained 30 or more components was laurax.

The existence of white smell makes sense, says neuroscientist Tim Jacob of Cardiff University, UK. Each receptor in our olfactory system detects a specific feature of a molecular odour. A cocktail of chemicals that activates all the receptors equally at once would saturate the brain’s capacity to identify a particular molecule.

Sobel envisages white smell being used to obscure unpleasant odours. And the whiff of white? Not odourless, he says, but “kind of nice”.